The present invention generally relates a polishing pad useful for polishing and planarizing substrates using a chemical-mechanical planarization (“CMP”) process. More particularly, the present invention provides a polymeric matrix polishing pad containing embedded polymeric capsules useful in conjunction with an in-situ optical end-point detection device.
In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting and dielectric materials are deposited on or removed from a surface of a semiconductor wafer. Thin layers of conducting, semiconducting, and dielectric materials may be deposited by a number of deposition techniques. Common deposition techniques in modern processing include physical vapor deposition (PVD), also known as sputtering, chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), and electrochemical plating (ECP).
As layers of materials are sequentially deposited and removed, the uppermost surface of the wafer becomes non-planar. Because subsequent semiconductor processing (e.g., metallization) requires the wafer to have a flat surface, the wafer needs to be planarized. Planarization is useful in removing undesired surface topography and surface defects, such as rough surfaces, agglomerated materials, crystal lattice damage, scratches and contaminated layers or materials.
In a typical CMP process, a lower platen having a circular rotating plate holds a polishing pad; the polishing pad is attached such that the polishing surface of the polishing pad faces up. A polishing composition, that typically contains chemistry that interacts with the substrate and may contain abrasive particles, is supplied to the polishing surface of the polishing pad. An upper platen having a rotating carrier holds a substrate; the substrate is held such that the surface to be planarized faces down. The carrier is positioned so that its axis of rotation is parallel to and is offset from that of the polishing pad; additionally, the carrier can be oscillated or otherwise moved about the surface of the polishing pad as is appropriate for the CMP process. The substrate and the polishing pad are brought into contact and forced together with downward pressure by the upper platen, whereby the polishing composition on the surface of the polishing pad is contacted with the surface of the substrate (the working environment), allowing the chemistry to react with the substrate, and mechanical polishing takes place.
Polishing pads can be manufactured in a variety of ways, such as casting a cake or by casting a sheet. In a typical manufacturing process, the polymer pad material ingredients, which may include one or more pre-polymers, cross-linking agents, curing agents and abrasives, are mixed, resulting in a resin. The resin is transferred to a mold by pouring, pumping or injecting etc. The polymer typically sets quickly and may finally be transferred to an oven for completion of the curing process. The cured cakes or sheets are then cut to a desired thickness and shape.
Polishing pad surface asperities aid in transporting the polishing composition during the CMP process and can be created on the polishing surface of the polishing pad in many ways. According to one method, surface asperities are created by embedding hollow polymeric capsules in a polishing pad comprising a polymeric matrix. Specifically, surface asperities are created by rupturing the capsules and exposing the hollow void contained therein to the working environment on the surface of the polishing pad. This may be accomplished by conditioning the polishing pad.
Typically, conditioning consists of abrading the polishing surface of the polishing pad with diamond points (or other scoring or cutting means) embedded in the conditioning surface of a conditioning pad. As the conditioned polishing pad is used, the asperities wear away and become clogged with debris from the CMP process. This results in the loss of polishing pad surface asperities with continued use. Asperities can be regenerated, as the polishing surface is worn during the CMP process, by continuous or intermittent conditioning. Asperities can also be regenerated during the polishing process, without abrasive conditioning, as the embedded polymeric capsules are exposed to the polishing surface and ruptured. For convenience, the term conditioning refers to regeneration of surface asperities whether through pad wear exposing new asperities, through the use of a conditioning pad or through other regeneration techniques.
In addition to transportation of the polishing composition, the polishing composition must flow over the surface of the polishing pad for the polishing process to be effective. This flow is aided by large-scale texture. Large-scale texture is created on the polishing surface of the polishing pad by the introduction of grooves. Groove pattern design and groove dimensions affect polishing pad characteristics and the CMP process characteristics. Polishing pad grooving is well known in the art, and known groove designs include radial, circular, spiral, x-y and others. Typically, grooves are introduced in the polishing surface of a polishing pad after it is formed through mechanical means such as cutting, using a fixed blade, such as a chisel, or other cutting means, but may be integrally formed in the pad, or created by stamping.
It is important to stop the CMP process when the desired amount of material has been removed from the surface of the substrate. In some systems, the CMP process is continually monitored throughout in order to determine when the desired amount of material has been removed from the surface of the substrate, without stopping the process. This is typically done by in-situ optical end-point detection. In-situ optical end-point detection involves projecting laser (or some other) light through an aperture or a window in the polishing pad from the platen side so that the laser light is reflected off the polished surface of the substrate and is collected by a detector. These systems work well for optically transparent polishing pads, but are typically not useful for filled pads.
A typical pad used in the CMP process is IC1000™ polishing pads manufactured and sold by Rohm and Haas Electronic Materials CMP Technologies. As illustrated in FIG. 1, these pads 10 have a clear matrix 12 and porosity formed from gas-filled polymeric spheres 14. The large difference between the refractive indexes of the clear matrix 12 and the polymeric spheres 14 corresponds to a large degree of refraction. This refraction, especially when large numbers of interfaces are encountered for high porosity polishing pads, creates opacity because light entering the polishing pad is substantially refracted and does not travel through the polishing pad with sufficient freedom to reflect back through the pad for effective signal generation.
FIG. 2 illustrates the optical path of a typical gas-filled sphere 14 of the prior art. The polymeric capsule 14 has a polymeric shell 16 having a first refractive index, a gas core 18 having a second refractive index, a first interface 20 where the polymeric shell 16 contacts the polymeric matrix material 12, and a second interface 22 where the polymeric shell 16 contacts the gas core 18. The refractive index of the gas core 18 differs from the polymeric shell 16 by an unacceptable amount for most commercial polishing equipment. Light ray 24 is shown traveling through the polymeric matrix material 12, where it encounters the first interface 20 and is refracted slightly. Light ray 24 travels through the polymeric shell 16 where it encounters the second interface 22 and is partially reflected (discussed more below) as shown by light ray 26, and partially refracted as shown by light ray 28. Light ray 28 travels through the gas core 18 until it contacts the second interface 22 a second time, where it is again partially reflected, shown as light ray 30, and partially refracted, as shown by light ray 32. Light ray 32 encounters the first interface 20, is slightly refracted, and exits the polymeric capsule 14 with a significant signal loss. Furthermore, reflected light ray 30 travels through the gas core 18 until it encounters the second interface 22 where it is partially reflected, shown as light ray 34, and partially refracted, shown as light ray 36.
One such window is disclosed in U.S. Pat. No. 5,893,796, to Birang at al, in which the window is made of a clear polymer and is inserted into an aperture formed in a polishing pad. The amount of light that is reflected from the surface of the substrate corresponds to the amount of material that has been removed. When the amount of light detected equals a predetermined value, the CMP process has reached the desired end-point and the CMP process is terminated.
The window of the '796 patent may be inserted into a formed pad in which an aperture has been made to receive the window, or alternatively the window may be cast in place. Any method of manufacturing a polishing pad with a window according to the '796 patent, however, results in a two (or more) piece polishing pad. As a result, polishing composition may enter into the seam between the polishing pad material and the window material, and may leak through the polishing pad, interfering with the in-situ optical end point detection apparatus. Many attempts have been made to reduce or eliminate this phenomenon, for example, by covering the bottom of the polishing pad with an impermeable film. This method, however, involves additional steps and new materials into the manufacturing process, which is inefficient and costly. In addition, the window material is frequently different from the polishing pad material, and has different characteristics than the polishing pad material, which may adversely affect polishing.
Hence, what is needed is a porous polishing pad that is transparent and allows inspection of the surface of the substrate without the need for a separate window.